1. Field of the Invention
This invention relates to a color video signal reproducing apparatus for reproducing a color video signal from a recording medium.
2. Description of the Related Art
In the conventional apparatus for recording and reproducing a color video signal, such as a video tape recorder of the kind equipped with a camera (hereinafter referred to as a camera-combined type VTR) which is capable of performing a reproducing operation as well as recording, a recording operation is arranged to be performed in the following manner: A luminance signal Y and color difference signals (R-Y and B-Y) are obtained from R (red), G (green) and B (blue) signals by means of a matrix circuit. The luminance signal Y is frequency modulated after a pre-emphasis process. Further, the color difference signals R-Y and B-Y are quadrature two-phase modulated into a carrier chrominance signal (hereinafter referred to as a chrominance signal). The chrominance signal is then frequency converted into a low-band chrominance signal. The luminance signal which is frequency modulated and the chrominance signal which is converted to a lower frequency band are then frequency multiplexed into a composite recording signal. The composite signal is recorded on a magnetic tape by a rotary magnetic head.
In reproduction, low-pass and band-pass filters are operated to separate the frequency modulated luminance signal and the low-band converted chrominance signal from each other. The frequency modulated luminance signal is demodulated back into the original luminance signal and the low-band converted chrominance signal is frequency converted into the original chrominance signal. The luminance and chrominance signals are multiplexed and are produced as a reproduced composite video signal.
Any drop-out that occurs in the reproduced composite video signal has been arranged to be compensated solely for the reproduced luminance signal on the side of the reproduction system of the VTR. FIG. 1 of the accompanying drawings shows by way of example and in outline the arrangement of the reproduction system of the conventional VTR which is provided with a circuit for correcting the drop-out of a luminance signal.
Referring to FIG. 1, rotary magnetic heads 1a and 1b reproduce a composite signal from a magnetic tape which is not shown. The reproduced composite signal is supplied to a head change-over switch 2. The switch 2 is arranged to have its connecting position switched from one side A over to another side B by means of a head change-over pulse which is generated according to the cycle of rotation of the magnetic heads 1a and 1b. The composite signal reproduced by the magnetic heads 1a and 1b are supplied to a reproduction amplifier 3 via the switch 2. If, for example, the signal corresponds to a television signal of the NTSC system, the position of the switch 2 is arranged to be changed every 1/60 sec.
The amplifier 3 amplifies the reproduced composite signal. The amplified signal is supplied to a high-pass filter (HPF) 4; a low-pass filter (LPF) 5 and a known frequency control (AFC) circuit 6. The HPF 4 separates a reproduced luminance signal and the LPF 5 a reproduced chrominance signal respectively.
The reproduced luminance signal is demodulated by a frequency demodulation circuit 7. The demodulated luminance signal is supplied to one horizontal scanning period (1H) delay circuit 9 and a mixer 10 via a change-over switch 8 which is normally in connection with one side L thereof. Meanwhile, the reproduced chrominance signal which is in a state of having been converted to a low frequency band is brought back to the original chrominance signal form by a known frequency conversion circuit 11 and a band-pass filter (BPF) 12. In other words, it is brought back into a signal modulated with a subcarrier frequency fsc (3.58 MHz). The AFC circuit 6, a phase control (APC) circuit 13, an auxiliary frequency conversion circuit 14 and a reference signal generating circuit 15 are arranged in a known manner to generate a time base corrected chrominance signal with a frequency converting process performed at the above stated frequency conversion circuit 11 in a known manner.
The luminance signal and the chrominance signal which have been processed in the above stated manner are added up at the mixer 10 into a reproduced composite video signal. In the event of occurrence of a drop-out, the conventional apparatus operates as described below:
When a drop-out occurs in the reproduced composite video signal in the arrangement shown in FIG. 1, the reproduced luminance signal at that part brings about a sudden change in the envelope of the signal. When the level change of the envelope is detected, a drop-out detection circuit 16 produces and supplies a drop-out detection pulse to the change-over switch 8.
Then, the change-over switch 8 is connected to its side H and remains in that connecting position over one horizontal scanning period (hereinafter referred to as 1-H) after receipt of this drop-out detection pulse. The luminance signal is thus delayed by a 1-H delay circuit 9 as much as one horizontal scanning period (1-H). As a result, the luminance signal which is obtained one horizontal scanning period before and is thus delayed is supplied to the mixer 10 via the switch 8. The luminance signal of the part where the drop-out has occurred is thus compensated for by the luminance signal which is obtained one horizontal scanning period before.
While the luminance signal has been thus arranged to be compensable for a drop-out, the chrominance signal has never been compensated for the drop-out, because:
In accordance with the conventional method of compensating for a drop-out, the drop-out signal portion is compensated by interpolation with another signal portion. This method necessitates use of a 1-H delay circuit or a 1-frame period delay circuit. However, as well known, the chrominance signal is modulated with the subcarrier frequency fsc which is in an interleaving relation to the repetition frequency fH of the horizontal scanning line. Therefore, a mere attempt to compensate the chrominance signal for a drop-out portion thereof simply with a 1-H period delayed chrominance signal portion would affect the continuity of the subcarrier fsc without some additional process of inverting the phase of the chrominance signal or the like. Besides, even a slight deviation of the timing of a delay action would cause a change in hue and thus would result in a completely different chrominance signal when the signal is reproduced.
Because of the above stated reason, unlike the luminance signal, the chrominance signal is hardly compensable for a drop-out by means of a delay circuit. As a result, no drop-out compensation has been made for a chrominance signal.
Further, the reproduced composite signal correcting processes to be carried out by the VTR in reproducing a video signal from a magnetic tape include a process of removing noises mixed in the reproduced composite video signal. The noise removing process has been accomplished only for a reproduced luminance signal and in a manner as described below:
FIG. 2 shows in outline and by way of example the arrangement of the reproduction system of a VTR which is provided with a noise removing circuit for removal of noises from the luminance signal. In FIG. 2, the parts which are the same or equivalent to those shown in FIG. 1 are indicated by the same reference numerals as FIG. 1.
Referring to FIG. 2, a reproduced composite video signal which is reproduced by rotary heads 1a and 1b from a magnetic tape (not shown) is supplied to a changeover switch 2. The connecting position of the switch 2 is arranged to be alternately shiftable between two sides A and B of the switch 2 by a head change-over pulse according to the cycle of rotation of the magnetic heads 1a and 1b. A composite video signal which is reproduced by the rotary magnetic heads 1a and 1b is supplied via the switch 2 to a reproduction amplifier 3. The switch 2 is arranged to have its position changed, for example, every 1/60 sec in the case of a video signal corresponding to the television signal of the NTSC system.
The signal amplified by the amplifier 3 is supplied to an HPF 4, an LPF 5 and an AFC circuit 6. A reproduced luminance signal is separated by the HPF 4 while a reproduced chrominance signal is separated by the LPF 5. The reproduced luminance signal is then demodulated by a frequency demodulation circuit 7. The demodulated luminance signal is supplied to a correlation detection circuit 17 and subtracters 18 and 19.
Meanwhile, a reproduced chrominance signal which is included in the composite signal is in a state of having been converted to a low band is brought back to its original chrominance signal form or converted with a subcarrier fsc (3.58 MHz) through a known frequency conversion circuit 11 and a known BPF 12. The reproduced chrominance signal which is thus processed back to its original state is supplied to a mixer 20. An AFC circuit 6, an APC circuit 13, an auxiliary frequency conversion circuit 14 and a reference signal generating circuit 15 are arranged in a known manner to give a time base corrected chrominance signal through a frequency converting process performed at the frequency conversion circuit 11.
A noise removing operation on the luminance signal demodulated by the frequency demodulation circuit 7 is as follows: The frequency demodulated luminance signal is supplied to a correlation detection circuit 17 which is included in the arrangement shown in FIG. 2. The circuit 17 is arranged to perform a computing operation for determining a correlation between the luminance signal and a luminance signal portion which is obtained one horizontal period (1-H) before and is delayed 1-H by a one-horizontal scanning period (1-H) delay circuit 21 after removal of noises. A setting value of a coefficient to be integrated at a coefficient integrating circuit 22 is then controlled according to the result of the above stated computing operation. In other words, the correlation detection circuit 17 is arranged to set a coefficient value .alpha. within a range of 0.ltoreq..alpha..ltoreq.1 to have it set at a value closer to .alpha.=1 or closer to .alpha.=0 according as the correlation is stronger or weaker. In the case where the correlation is determined to be none, the setting value is .alpha.=0.
After determination of the correlation between the present luminance signal portion and the previous luminance signal portion preceding by 1-H and after setting of the coefficient .alpha., the luminance signal portion delayed by the 1-H delay circuit 21 and the present luminance signal portion are subjected to a subtracting operation performed by the subtracter 18. A difference signal thus obtained is supplied to the above stated coefficient integrating circuit 22. The coefficient .alpha. which is set in the above stated manner is then integrated by the circuit 22. The result of the integrating operation is supplied to another subtracter 19.
Meanwhile, the present portion of the reproduced and frequency demodulated luminance signal is supplied to the subtracter 19. The subtracter 19 subtracts the output of the coefficient integrating circuit 22 from the present portion of the luminance signal.
In other words, the difference signal produced from the subtracter 18 can be assumed to have a noise component in the event of strong correlation between the present luminance signal and the 1-H delayed preceding luminance signal portion. In that event, therefore, the coefficient .alpha. is set close to 1 to have the difference signal of the subtracter 18 subtracted almost in its entirety from the present luminance signal. In case of weak correlation, the difference signal produced from the subtracter 18 is assumed to have little noise component therein. In this case, therefore, the coefficient .alpha. is set at a value closer to 0 and the present luminance signal is allowed to be produced with little of the difference signal of the subtracter 18 subtracted from the present luminance signal. With the noise component thus removed therefrom, the luminance signal is supplied to the 1-H delay circuit 21 and the mixer 20. Then, at the delay circuit 21, the signal is delayed for 1-H for detection of the above stated correlation and for noise removal. At the mixer 20, the luminance signal from which noises are removed and the chrominance signal which has been frequency converted in the above stated manner are multiplexed and are produced as a reproduced composite video signal.
While noises have been removed from the luminance signal in the manner described above, this noise removing operation has not been performed on the chrominance signal because: The noise removing operation is performed by using a previous signal correlating with a present signal. The conventional method necessitates use of a 1-H delay circuit or a 1-frame period delay circuit. However, as well known, the chrominance signal is modulated by the subcarrier fsc which is in an interleaving relation to the repeating frequency fH of horizontal scanning lines. Therefore, mere arrangement to perform the above stated noise removing operation, by simply performing a subtracting or adding operation on the present and previous chrominance signal portions would impair the continuity of the subcarrier fsc unless some phase inverting process or the like is performed as the chrominance signal has its phase inverted every 1-H period. Further, any deviation of signal phase due to inconstancy of delay time of the delay circuit results in a change of hue. Then, it becomes impossible to have color information reproduced with fidelity.
It has been thus extremely difficult, unlike in the case of the luminance signal, to remove noises from the chrominance signal by the method of using a delay circuit for utilization of the correlativity of signals. Therefore, the above stated noise removing process has not been applied to the chrominance signal. Any other correction process that would vary the signal phase besides the noise removing process also has not been performed.
In accordance with the conventional method as described in the foregoing, correction of color information during reproduction must be performed in the form of the chrominance signal. However, the chrominance signal has some hue information included also in the phase thereof. It is, therefore, difficult to carry out correction while retaining the continuity of the phase variations of the chrominance signal. Correction such as compensation for a drop-out of the signal and noise removal by means of a feedback type noise removing circuit cannot be carried out by any correcting method that disturbs the phase relation. The deterioration of reproduced color information such as inConstant colors thus has been left uncorrected because of the problem described.